[0001] The present invention relates to a method of controlling a mold opening/closing operation
in an injection molding machine having a toggle-type mold-clamping mechanism.
[0002] Conventionally, for controlling the speed of a movable plate of the mold or the pressure
applied to the movable plate when a mold of an injection molding machine is opened
or closed, a method has been employed in which the speed or the pressure is changed
depending on the displacement position of the movable plate. A limit switch, potentiometer,
or linear encoder has been used to detect the displacement position. Especially when
the mold opening and closing operation is centrally and remotely controlled, there
has been employed a controlling method wherein a displacement position detecting device
such as a potentiometer or a linear encoder is used to detect directly the displacement
position of the movable plate, and a signal is thereby generated to change the speed
or pressure when the detected value agrees with a preset value.
[0003] This method is frequently employed not only in direct-pressing type mold-clamping
systems, but also in toggle-type mold-clamping systems. Although enhancement of the
mold-clamping force just before a mold-clamping operation is completed is brought
about by a toggle-type mold-clamping mechanism. We have found that it is possible
to increase the accuracy in detecting foreign matter sandwiched between the mold parts
which are clamped together, if the control is effected by detecting the displacement
distance of a rod of a mold-clamping hydraulic cylinder which operates the toggle
mechanism by detecting the displacement position of the movable plate. That is because
the displacement distance S (stroke) of the rod is much greater than the displacement
distance S' (stroke) of the movable plate and thus the accuracy is correspondingly
higher if control is performed in accordance with the displacement distance of the
rod. The ratio between the displacement distances (S/S') is typically more than about
ten, and may be more than about 20 just before the mold parts come into contact with
each other.
[0004] In the toggle-type mechansim, there is a correlation between the displacement distance
S of the rod and the displacement distance S' of the movable plate. However, as indicated
by the graph of figure 2 of the accompanying drawings, the relationship between the
displacement distances S and S' is non-linear, and therefore the above-mentioned method
in which the displacement distance of the movable plate is directly detected to control
the speed or the pressure in the mold opening and closing operation has generally
been preferred, similar to that used in the direct pressing type mold-clamping system.
In a mold opening/closing control method which employs the advantages of a toggle-type
mold-clamping system, there has been additionally provided a detecting device such
as a limit switch, a contactless switch, or the like, for detecting the displacement
distance S of the rod of the mold-clamping cylinder in order to increase the accuracy
in detecting foreign matter. However, such a detecting device must be provided not
as a central and remotely controlled unit, but as a unit inside a mold-clamping device
part of the injection molding machine.
[0005] Therefore, because in conventional injection molding machines having a toggle-type
mold-clamping mechanism the displacement distance of the movable plate is directly
detected and controlled in order to control the speed of or pressure applied to the
movable plate in mold opening/closing operations, and further because it is necessary
to provide another device for detecting the displacement distance of the rod of the
mold-clamping cylinder inside the machine, the overall structure of the apparatus
is complicated. Furthermore, in toggle-type injection molding machines, it has been
proposed to provide cam means and limit switches for detecting an operational movement
of a cross link to control the mold opening/closing operation. However, according
to this proposal, since the mold movement and the cross link movement are not kept
in a linear relationship, it is difficult to effect adjustment and setting of the
control mechanism.
[0006] In order to solve the problems of the prior art described above the present invention
utilizes the fact that there is a correlation between the displacement distance S
of the rod of the mold-clamping cylinder and the displacement distance S' of the movable
plate.
[0007] Thus, according to the present invention, a method of controlling the opening and
closing of a mold, which includes a movable plate, in an injection molding machine
having a toggle-type mold-clamping mechanism which is operated by hydraulic pressure
is characterized by the steps of: providing a changeover position setting device for
setting one of a speed of the movable plate and a changeover position of a hydraulic
pressure supply to the mechanism to a valve in accordance with a corresponding position
of the movable plate, arithmetically converting a value set in the setting device
into a corresponding displacement distance of a rod of a mold-clamping cylinder of
the mechanism; storing the converted value; detecting a displacement distance of the
rod; comparing the stored converted value with the detected displacement distance;
and changing the hydraulic pressure when the detected displacement distance of the
rod and the stored value coincide with each other.
[0008] More specifically and preferably, when the value of the detected displacement distance
of the rod of the mold-clamping cylinder agrees with the value in the converted changeover
position storage device, an electromagnetic flow adjustment valve or an electromagnetic
relief valve in a hydraulic pressure circuit is controlled by a sequencer.
[0009] Thus it will be seen that the displacement distance S' of the movable plate is converted
into the displacement distance S of the rod of the mold-clamping cylinder and control
is performed by comparing the result of the conversion with the detected value of
the actual displacement distance of the rod.
[0010] An example of a method in accordance with the invention will now be described with
reference to the accompanying drawings in which:-
Figure 1 is a block diagram showing a preferred embodiment of a control system for
carrying out the method in accordance with the present invention; and,
Figure 2 is a graph showing the relationship between the displacement distance of
a rod of a mold-clamping cylinder and the displacement distance of a movable plate
forming parts of a mold opening and clamp mechanism.
Figure 1 is a block diagram illustrating a preferred embodiment of a control system
for carrying out the method according to the present invention.
[0011] A rod displacement distance detecting device 7 for detecting the displacement distance
S of a rod 6a of a mold-clamping cylinder 6 is provided between a mold-clamping housing
2 and a cross link 5 connected to the rod 6a. Reference numeral 1 designates a movable
plate, 3 a toggle link, and 4 a tie bar for guiding the movable plate 1.
[0012] The control system will be described. Reference numeral 11 designates a changeover
position setting device for controlling the speed of or pressure applied to the movable
plate 1 in a mold opening/closing operation, the changeover position being determined
depending on the displacement position of the movable plate. An arithmetic conversion
arithmetic device 10 converts the setting value of the changeover position setting
device 11 into a displacement distance S of the rod 6a according to a mathematical
expression corresponding to the relationship between the displacement distance S of
the rod 6a and the displacement distance S' of the movable plate 1 (which have been
stored in advance), and supplies the converted value to a converted changeover position
storage device lla. A comparator 9 is supplied with the detected value from the device
7 for detecting the displacement distance of the rod 6a and a converted value of the
setting value from the converted changeover position storage device lla. When the
values of the two agree, the comparator 9 provides a changeover signal to a sequencer
12. In response to this signal, the sequencer 12 actuates an electromagnetic flow
adjusting valve or an electromagnetic relief valve 14 in a hydraulic circuit to operate,
through a driver amplifier 13, a valve to control the above-mentioned speed or pressure.
[0013] Since the present invention employs an arrangement as described above, it is not
necessary to detect directly the displacement position of the movable plate, and the
displacement position of the movable plate can be detected as an amplified value of
the displacement position of the mold-clamping cylinder. Hence, the accuracy of changeover
control is increased, and it is possible to make the setting of the changeover position
on the basis of the displacement position of the movable plate. Further, the accuracy
in detecting foreign matter in the mold is improved by using the toggle magnifying
ratio, which is a feature of a toggle-type mold clamping mechanism, and all of the
control operations described above may be performed centrally and remotely.
1. A method of controlling the opening and closing of a mold, which includes a movable
plate (1), in an injection molding machine having a toggle-type mold-clamping mechanism
which is operated by hydraulic pressure, characterised by the steps of: providing
a changeover position setting device (11) for setting one of a speed of the movable
plate (1) and a changeover position of a hydraulic pressure supply to the mechanism
to a valve in accordance with a corresponding position of the movable plate (1), arithmetically
converting a value set in the setting device (11) into a corresponding displacement
distance of a rod (6a) of a mold-clamping cylinder (6) of the mechanism; storing the
converted value; detecting a displacement distance of the rod (6a); comparing the
stored converted value with the detected displacement distance; and changing the hydraulic
pressure when the detected displacement distance of the rod and the stored value coincide
with each other.
2. A method according to claim 1, wherein the step of changing the hydraulic pressure
is effected with a sequencer (12) driving a flow adjusting value.
3. A method according to claim 1, wherein the step of adjusting the hydraulic pressure
is effected with a sequencer (12) driving an electromagnetic relief valve (14).
4. A method according to claim 1, substantially as described with reference to the
accompanying drawings.